1. Technical Field
The present invention relates to an electric power steering system for controlling steering of a vehicle and, more particularly, to a fail-safe.
2. Background Art
In electric power steering system for controlling steering of a vehicle, control is sometimes suspended by its fail-safe function when any trouble occurs. This control suspension secures steering performance of the vehicle by suspending turning operation of steering wheel with a motor and switching the operation to manual operation mode by the driver. As a specific example of the control suspension, an electric current flowing to the motor which outputs an auxiliary torque is interrupted not only by suspending a motor driving circuit but also by opening a relay for supplying a power in order to interrupt the power supply. However, there is a possibility that contact point of this relay is welded and does not open, and therefore it is necessary to inspect the function of the relay. Japanese Patent No. 2715473 proposes a method for detecting the relay welding.
Described below is such a problem of relay welding with reference to FIG. 8. In the drawing, reference numeral 1 is a torque sensor for detecting a steering torque. Numeral 2a is means for calculating controlled variable, which calculates an auxiliary torque on the basis of the torque information and outputs a control signal, and is a microcomputer (hereinafter referred to as CPU). Numeral 13 is an interface circuit 1 which transmits this-torque information to the CPU 2a. Numeral 3 is a motor which is arranged in the steering system and generates the auxiliary torque, and is driven by switching elements 4 to 7. Numeral 8 is a first driving circuit mainly composed of the switching elements 4 to 7. Numeral 9a is a second driving circuit for driving the switching elements 4 to 7 according to a control signal from the CPU 2a. Numeral 10a is a voltage monitoring circuit which detects voltages at two ends of the motor 3, and the outputs therefrom are inputted to the CPU 2a as E1 and E2. Numeral 20 is a relay which supplies a power to the motor 3 and the switching elements 4 to 7, and is driven by a switch driving circuit 21 depending on a signal from the CPU 2a. 
Described now is how to detect welding of the relay 20. First, contact point of the relay 20 is opened in advance by the switch driving circuit 21. The switching element 4 or 5 is driven, and the CPU 2a inputs the result from the voltage monitoring circuit 10a. In this case, the voltage monitoring circuit is an inverter circuit comprising a transistor element, and each driving mode thereof is shown in FIG. 9.
When the switching element 4 is turned on and the element 5 is turned off, both of the signals of E1 and E2 are high (hereinafter referred to as H) under normal condition. All other cases indicate an abnormal condition. In the same manner, when the switching element 4 is turned off and the element 5 is turned on, both of the signals of E1 and E2 are H under normal condition, and all other cases indicate an abnormal condition. FIG. 9 clearly shows that the contact point of the relay is welded except under normal condition. However, the driving circuit 9a, voltage monitoring circuit 10a, and others are under normal condition.
On the other hand, Japanese Patent Publication (unexamined) No. 62-231871 proposes that a relay is interposed also between the driving circuit and the motor (hereinafter referred to as motor relay). FIG. 10 is a block diagram in which a motor relay 12 is connected in series to the motor 3. Diodes are inserted in parallel between the switching elements 4 to 7. Even if the relay 20 (hereinafter referred to as power supply relay) is opened, the motor is rotated when the driver turns the steering wheel, and a regenerative current flows by means of these diodes. Therefore, in case where one of the switching elements is short-circuited and broken, there arises a condition of different load in the turning direction of the steering wheel depending on the turning direction of the steering wheel or the motor, and this obstructs smooth turning of the steering wheel. To cope with this, the motor relay 12 is added.
It may be obvious to combine the foregoing two inventions, however, actual control unit (hereinafter referred to as ECU) is further provided with a fail-safe function, and this brings about a problem in that it is impossible to easily inspect the motor relay. For example, in consideration of any trouble in the CPU 2a, means for inhibiting drive of the motor (hereinafter referred to as motor drive inhibiting means) is required other than the CPU. Such motor drive inhibiting means has made it difficult to inspect the relay. In other words, it is certainly possible to inspect the motor relay so long as the motor drive inhibiting means is in a motor driving condition. But when the motor drive inhibiting means is in a motor drive inhibiting condition, the switching elements are not driven even if the motor relay is inspected and it is impossible to judge whether or not the motor relay is in malfunction.
This problem is now described in more detail with reference to FIG. 10. Drive inhibition circuit 18g acts so that the driving circuit 9a may become inoperative in a case where the steering wheel is little turned, i.e., when torque is little produced. A switching element for left turning is suspended during a clockwise turning, while a switching element for right is suspended during a counterclockwise turning. FIG. 11 is a graph showing the relation between the torque and the output of the torque sensor. The switching element 4 or 7 for clockwise turning is suspended when the torque is smaller than Tq2, and the switching element 5 or 6 is suspended when the torque is larger than Tq1. Less torque is produced in a range from v2 to v1 in torque sensor output voltage. It is therefore unnecessary to drive the motor 3 in this range. Accordingly, the drive inhibition circuit 18g, which acts so that the driving circuit 9a may become inoperative, is added in addition to the CPU 2a. More specifically, the drive inhibition circuit 18g in FIG. 10 judges whether or not the torque sensor output voltage is within v1 and v2 through comparison using two comparators 18b, 18c. 
When the torque sensor output is larger than v1, a transistor 18d is turned on and the switching element 5 or 6 is turned off. On the other hand, when the torque sensor output is smaller than v2, the circuit is connected so that the transistor 18e may be turned on and the switching element 4 or 7 may be turned off. When the drive inhibition circuit 18g is working, i.e., when the transistor 18d or 18e is turned on, the switching elements 4, 5 or 6, 7 still remain in the off condition even if the CPU 2a outputs a control signal. Therefore, it is impossible to inspect the motor relay 12 in this condition.
As described above, the conventional system has a problem in that the drive inhibition circuit obstructs inspection of the relay and the relay cannot be easily inspected.
The present invention was made to resolve the above-discussed problems and has an object of providing an electric power steering system in which the fail-safe function is improved making it possible to detect a trouble.
An electric power steering system according to the invention, which possesses torque detecting means for detecting a steering torque, controlled variable calculating means for calculating a controlled variable, by which a motor conducts steering of a vehicle, on the basis of the torque information from the torque detecting means, and driving means for driving the motor by switching elements upon receiving the controlled variable, comprises: drive restricting means for restricting drive which restricts at least a part of operation of the driving means on the basis of the torque information separately from the controlled variable calculating means; first switching means which is arranged between the motor and drive means for connection and disconnection; and voltage monitoring means for monitoring terminal voltage of the motor;
wherein the controlled variable calculating means operates at least a part of said driving means for driving and inspects function of the first switching means on the basis of voltage information from the voltage monitoring means in a case where it is judged that the drive restricting means is suspending its operation or that the function is partly restricted on the basis of magnitude and direction of the torque or condition of the drive restricting means.
As a result, the system has an advantage of precisely inspecting the function of the first switching means even if the system possesses the drive restricting means.
It is preferable that the controlled variable calculating means is connected with the drive restricting means and inspects the function of the first switching means in a case where the operation of the drive restricting means is suspended or partly restricted on the basis of information from the drive restricting means. As a result, the system has an advantage of easily inspecting the function of the switching means.
An electric power steering system according to the invention, which possesses torque detecting means for detecting a steering torque, controlled variable calculating means for calculating a controlled variable, by which a motor conducts steering of a vehicle, on the basis of torque information from the torque detecting means, and driving means for driving the motor by switching elements upon receiving the controlled variable, comprises: drive inhibiting means for inhibiting drive which inhibits at least a part of operation of the driving means on the basis of the torque information separately from the controlled variable calculating means; first switching means which is arranged between the motor and drive means for connection and disconnection; voltage monitoring means for monitoring terminal voltage of the motor; and invalidating means for temporarily invalidating operation of the drive inhibiting means;
wherein the controlled variable calculating means operates the driving means and inspects function of the first switching means on the basis of voltage information from the voltage monitoring means during the period when the invalidating means is suspending the operation of the drive inhibiting means.
As a result, the electric power steering system according to the invention has an advantage of precisely inspecting the function of the first switching means even if the system possesses the drive inhibiting means.
It is preferable that, in the electric power steering system according to the invention, the invalidating means invalidates the drive inhibiting means by controlling a power supply of the torque detecting means.
As a result, the electric power steering system according to the invention has an advantage of easily invalidating the drive inhibiting means at any time.
It is also preferable that, in the electric power steering system according to the invention, the invalidating means invalidates the drive inhibiting means by controlling a signal from the torque detecting means.
As a result, the electric power steering system according to the invention has an advantage of easily invalidating the drive inhibiting means at any time.
It is also preferable that, in the electric power steering system according to the invention, the invalidating means invalidates the output signal of the drive inhibiting means by controlling the drive inhibiting means.
As a result, the electric power steering system according to the invention has an advantage of easily invalidating the drive inhibiting means at any time.
It is also preferable that, in the electric power steering system according to the invention, the function of the first switching means is inspected by inspecting whether the first switching means is capable of being disconnected and/or connected based on the relation between the drive of the switching elements of the driving means and the voltage monitored by the voltage monitoring means.
As a result, the electric power steering system according to the invention has an advantage of easily inspecting the function of the switching means.
It is also preferable that, in the electric power steering system according to the invention, the system has second switching means for supplying or disconnecting a power to the motor, and the controlled variable calculating means inspects whether the second switching means is capable of being disconnected and/or connected.
As a result, the electric power steering system according to the invention has an advantage of easily inspecting the function of the second switching means.
It is also preferable that, in the electric power steering system according to the invention, the system has second switching means for supplying or disconnecting a power to the motor and pseudo power supply means for supplying a pseudo power supply which is connected in parallel to the second switching means, and in a case where the controlled variable calculating means inspects the function of the first switching means, a power source is supplied from the pseudo power supply means.
As a result, the electric power steering system according to the invention has an advantage of adding electric current restriction and preventing induction of a secondary trouble even in a case where a trouble occurs downstream the second switching means.
It is also preferable that, in the electric power steering system according to the invention, the system has alarming means for alarming the driver in a case where the function of the first switching means is judged abnormal.
As a result, the electric power steering system according to the invention has an advantage of alarming the driver of the trouble or failure.